1. Field of the Invention
The present invention generally relates to an image forming apparatus such as a printer, a copying machine, a facsimile, etc., which forms an image on a recording medium such as paper, an OHP sheet, cloth, etc., and to an image fixing apparatus and an image fixing method in which the image on the recording medium is fixed by heat and pressure in a nip press region between a fixing member and a pressing member. The present invention more particularly relates to an image forming apparatus, an image fixing apparatus, and an image fixing method capable of effectively controlling an image fixing temperature.
2. Discussion of the Background
A background image forming apparatus such as a printer, a copying machine, a facsimile, etc. mostly adopts an electrophotographic process from the considerations of speed, image quality, cost, etc. The electrophotographic process includes operations of charging a photo-conductor, forming an electrostatic image by irradiating, developing to form a toner image, transferring the toner image onto a recording medium, and fixing the toner image on the recording medium.
Accordingly, such a kind of image forming apparatus using an electrophotographic process is equipped with an image fixing apparatus for fixing the toner image. As the fixing apparatus, a hot pressing type apparatus is mostly adopted from the consideration of safety. The hot pressing type fixing apparatus has a fixing nip press region between a fixing member, which is a drum or a belt to be heated, and a pressing member, which is also a drum or a belt for pressing. The toner image on a recording medium is fixed by heat and pressure in the nip press region.
FIG. 1 illustrates a cross sectional view of an exemplary configuration of a background fixing apparatus. A rotating fixing member 1 includes a heater 2 inside. A temperature sensor 3 is provided on the surface of the rotating fixing member 1. A rotating pressing member 4 presses the rotating fixing member 1 to form a fixing nip press region n. When a recording medium P passes through the fixing nip press region n, a toner image may be fixed by heat and pressure on the recording medium P. FIG. 2 illustrates another cross sectional view of the fixing apparatus of FIG. 1. The detected result by the temperature sensor 3 is input into a central processing unit (CPU) 7 through an input circuit 6. The CPU 7 controls the heater 2 through a driver 8 based on the detected result by the temperature sensor 3.
When a recording medium p2, which has a narrower width than a recording medium p1, is continuously fed, non-feeding areas m on both sides in the longitudinal direction may have a high temperature because heat in these areas is not absorbed in the recording medium p2. This may cause a problem. The problem is a decrease of the fixing apparatus life due to the higher temperature of the ends of the rotating fixing member 1 that exceeds an allowable temperature limit.
To avoid the problem, some image forming apparatuses have a fixing apparatus in which a fixing roller has two heaters, one for heating the center part of the fixing roller in the longitudinal direction, and the other for heating the end parts of the fixing roller. When using the narrower recording medium, only the heater for heating the center part of the fixing roller in the longitudinal direction is turned on, so the temperature of the ends of the rotating fixing member may rise but stays below the allowable temperature limit.
To avoid the problem in other examples, some image forming apparatuses have a fixing apparatus in which a temperature of the end parts of the fixing roller is detected, and a feeding timing of the recording medium may be controlled based on the detected temperature. For example, if the end parts of the fixing roller are expected to have a high temperature, the gap between the conveying recording mediums may be longer than a normal gap, and then the temperature of the ends of the rotating fixing member may rise but still be less than the allowable temperature limit.